Stretching Device and Method for Stretching a Plastic Film in the Transport Direction Thereof

ABSTRACT

The invention describes a stretching device for stretching a plastic film in its transport direction, having a first roller that can be driven by a first drive and can be rotated at a first rotational speed, and having a second roller that can be driven by a second drive and can be rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film.The device is characterized by means of varying the amount of air between the plastic film and flat elements of the first roller and/or the second roller on which the film rests.

The invention relates to a stretching device and a method for stretching a plastic film in its transport direction.

Such a stretching device is used to specifically influence the properties of a plastic film. In particular, the orientation of the molecules contained in the plastic film (hereinafter also referred to as “film”) is changed by such stretching. In order to achieve stretching, the stretching device comprises a first roller that can be operated by a first drive and can be rotated at a first rotational or circumferential speed. Furthermore, the stretching device comprises a second roller that can be operated by a second drive and rotated at a second rotational or circumferential speed. In the transport path of the plastic film, the second roller is arranged further downstream than the first roller.

The stretching of the plastic film is achieved by the fact that the second rotational speed is greater than the first rotational speed. In the context of stretching, rotational speed means the rotational speed that the film feels. This can, therefore, also be referred to as the circumferential speed of the first roller and the second roller.

However, stretching can also cause various problems. For example, monomers often adhere to the plastic film after stretching, which are emitted during the production of the plastic film as gas, or dust which is pressed into the film during stretching. Also, the heat transfer between film and roller often is not satisfactory. Furthermore, the properties of the film, especially geometric properties such as the thickness profile, can change in a direction transverse to the transport direction of the film, and also the flatness of the film can change as a result of the stretching.

It is, therefore, the task of this invention to propose a stretching device with which at least some of the above-mentioned problems can be reduced.

In accordance with the invention, this task is solved by all the features of claim 1. Possible embodiments of the invention are indicated in the dependent claims.

In the first aspect of the invention, a generic stretching device for stretching a plastic film is provided. This stretching device is characterized by means of varying the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film is placed. This invention thus makes it possible to vary the amount of air between the plastic film and the surface elements on which the film lies. This includes the reduction of the air volume, as well as the increase of the air volume. The plastic film runs onto the respective roller at an impact edge and leaves this roller again at a run-off edge. The impact edge and the release edge together with the side edges define the total surface element on which the film can rest at most. However, the actual surface element on which the film rests can be smaller than the total surface element.

By changing the air volume, the problems can be specifically avoided. For example, by increasing the air volume, the imprinting of dust or monomers into the film can be reduced. By reducing the amount of air, the transfer of heat between the roller and the plastic film can be significantly improved, as air can only absorb a small amount of heat and is thus a poor thermal conductor. A reduced amount of air, therefore, improves the overall heat transfer.

A reduction in the amount of air can also serve to make the film adhere more strongly to the roller so that the film is deprived of degrees of freedom to change geometrically. In particular, it can be achieved that the tendency of the thickness profile to change transversely to the direction of transport is reduced. A deterioration of the flatness of the film can also be reduced by measure in accordance with the invention.

It can be arranged that the means for changing the amount of air are positioned only in the edge areas of the roller. This prevents the usable effective width of the film from running on a part of the roller that is as flat as possible, and only the edge areas of the film adhere. Since these edge areas are often separated later, negative influences of the means for changing the air volume on the film edge areas do not have a negative effect here, but the advantages of the invention remain.

In order to be able to reduce the amount of air, it is provided in a first variant of the invention that the surface of the first roller and/or the second roller is provided with depressions. This reduces the actual size of all surface elements on which the film rests. Furthermore, areas at the edges of these surface elements are recessed relative to these surface elements, and thus provide volume elements into which the air can penetrate. Only a fraction of the air now remains between the surface elements on which the film rests and the film itself. In this context, it should be explained that the word “rests” is not to be equated with the fact that the film and the surface area are in full contact. A very small amount of air can remain in places or even over the entire surface, so that although macroscopically there is full-surface contact, microscopically there is not full-surface contact. The described measure ensures that the film adheres more strongly to the roller, thereby achieving the advantages described above.

Annular or spiral grooves can also be depressions in the sense described in the previous section. Spiral-shaped grooves can be single-start or multi-start, whereby multi-start grooves can have the same and/or an opposite direction of rotation (cross grooves). Such grooves can take up a quantity of air in the areas with which the film is in contact and release it back into the environment in the areas where the film is not in contact. The widths and/or depths of such grooves can be less than 2 mm, preferably less than 1 mm, and less than 0.5 mm. The upper transition between the surface elements on which the film rests and the grooves can be rounded or provided with bevels to avoid damage to the film.

In another variant of the invention, the means for varying the amount of air comprise through-holes in the surface of the first roller and/or the second roller. The through-holes constitute a fluid-communicating connection between the outer surface and a cavity within the first roller and/or the second roller. Such through-holes allow at least part of the air trapped between the film and the flat elements of the respective roll on which the film rests to be discharged into the cavity of the roll. Consequently, a reduced amount of air remains between the film and the surface elements, so that the film adheres more strongly to the surface elements of the roller surface and shows the undesired effects to a reduced extent or not at all.

In an advantageous embodiment of the invention, it is intended that the means for changing the amount of air comprise passage openings in the surface of the first roller and/or the second roller, wherein at least a part of the passage openings can be provided by a porous material, preferably a microporous material. This makes it possible to create many small channels which serve as passage openings. This has the advantage that the openings on the outer surface of the roller are small, and their edges do not have any negative effects on the film, for example in the form of impressions. Thus, the task described above is solved without having to accept new disadvantages. The porous material can form the shell of the roller or at least be a component thereof. The average pore size of the porous material can be between 5 and 100, 10 and 60, and preferably 20 and 45 micrometers. In one variant of the invention, it is intended that a porous material, in particular a microporous material, is only an outer coating of an otherwise closed roller surface. For example, a closed tube, e.g. of metal, may be provided onto which the porous material is or will be applied. The cavities provided by the porous material can now absorb the air that is between the film and the roller.

Irrespective of whether the porous material provides through openings or only cavities for air absorption, it may be intended to vary the pore size in the axial direction of the roller. At the edges of the roller shell, the volume per surface element provided by the porous material may be greater than in the central area, so that the film adheres more strongly to the roller at the edges.

The outer surface of the roller, which is at least partially formed by porous material, can be provided with a coating to prevent damage to the film, which has a lower hardness than the porous material.

Porous material can be sintered material, especially sintered metal or sintered plastic. A sintered material is a material that has been produced by a sintering process. In this process, fine-grained materials, which may be ceramic or metallic or may comprise plastic, are heated—often under increased pressure—but the temperatures remain below the melting temperature of the main components so that the shape of the workpiece is retained. For example, the shape can be sleeve-shaped, creating a sleeve that can later be applied to a base body. The base body and sleeve can then form the roller. The production of such a sleeve can also be carried out in an additive manufacturing process, whereby a three-dimensional body can be produced from a powdery raw material by local heating. The granularity of the material results in channels remaining between the grains, which form passage openings. The circumferential surface of a roll produced in this way may additionally be ground and/or polished to prevent damage to the film. However, to avoid an unintentional closure of the through-holes by abrasion, the surface of the roll can subsequently be treated by means of chemical milling. The through-holes are usually not designed in a straight line, but this does not detract from the ability to discharge air into the interior of the roll. A sintered material is usually a very hard material, so despite the high tensile forces within a stretching device, there is only a slight deflection, so the geometric properties of the film are hardly affected. In addition, the production of a sintered material is usually relatively inexpensive.

According to an alternative or additional embodiment of the invention, to reduce the amount of air, it is provided that the means for changing the amount of air comprise through-holes in, at least, a part of a roll shell of the first and/or the second roll, wherein at least a part of the through-holes is formed by bores. A roll shell can thereby comprise the roll surface, so that bores consequently extend from the surroundings of the roll into the roll interior. It is also possible to form the roll shell in several layers. For example, an inner part of a roll shell can be provided, which comprises bores. An outer part of the roll shell may comprise a porous, a microporous material. In this way, marking the edges of a hole on the film can be avoided, which would render the film unusable.

The holes can be smaller than 2 mm, preferably smaller than 1 mm, and smaller than 0.5 mm. The tops of the holes to the surface of the part of the roll shell can be rounded or conical. This measure again avoids damage to the film. A plurality of bores can be evenly distributed over the roll shell, whereby a certain number of bores can be provided per surface element. However, the distances between two adjacent holes can differ. It is particularly advantageous if the number of holes per surface element or the number of through-holes per surface element is varied, especially in the longitudinal direction of the roll shell, i.e. in the direction of the axis of rotation. Particularly at the edges of the roll shell, the number of holes or through-holes per surface element can be greater than in the central area, so that the film adheres more strongly to the roll at the edges. The edges can take up one-third of the roll shell length each so that the middle area also comprises one-third. However, the middle area can also be larger, so that the edges are correspondingly smaller. By varying the number of holes per surface element or the number of through-holes per surface element, the volume flow that passes through the wall of the roller can generally be varied. Such a variation can alternatively or additionally be achieved with sintered materials by varying the layer thickness of the sintered material so that there are different resistances for the volume flow.

In an advantageous embodiment, it is conceivable that the first and/or second roller is constructed in multiple layers, whereby in particular one layer can be designed to be displaceable relative to a second layer, whereby both layers can be detachably connected after a relative displacement. This makes it possible, for example, for an outer layer adapted to the properties of the film to be slid onto a base body. This embodiment is also advantageous to only have to replace the outer layer when the roller becomes dirty. Such a layer can then be cleaned, while the production of the film can be continued with another outer layer.

In a further variant of the invention, the means for changing the air volume can comprise at least one cavity which can be subjected to a reduced air pressure compared to the ambient pressure. It is thus achieved that a part of the air volume, which is supplied to the cavity of one of the rollers, can be discharged from the cavity, for example via passage openings. In this way, the air that is repeatedly introduced by the movement of the film and the roller can be continuously discharged, so that a stationary state can be set with regard to the reduced air quantity between the plastic film and surface elements of the first roller and/or the second roller on which the film rests. One possibility for pressurizing the cavity is a rotary joint via a journal of the respective roller. A hose or pipe can be connected to this rotary union, which connects the cavity with a vacuum source, in particular a pump, in a fluid-communicating manner.

Furthermore, it can be provided that at least one cavity can be pressurized to a higher pressure than the ambient pressure. This makes it possible to remove dirt from through-openings by means of a blow-out process. Additionally, or alternatively, a suction device can be provided, which is arranged in an angular area of the first and/or the second roller, in which no film is lying, i.e., between the peel line and the leading edge of the film. In this way, it is even easier to remove dirt from the surface of the roller. Such a suction device can extend at least partially in the direction of the roll axis. It can also have several separate suction chambers; the sizes of which can also be changed so that the suction device can be charged with different suction capacities. In areas in the direction of the roller which do not encounter the film, the suction power can be lower than in the other areas, as there is usually less contamination to be feared here. Contamination is often caused by paraffin waxes adhering to the film and being released onto the rollers.

According to an advantageous embodiment of the stretching device, according to the invention, it is provided that the first and/or the second roller comprises at least one cavity that is divided into at least two segments in the axial direction and/or in the circumferential direction, by means of at least one separating element, wherein at least one segment can be subjected to an air pressure which is reduced or increased compared to the ambient pressure. Each segment can thus be subjected to different air pressures, so that, for example, in the axial direction of the roller, the amount of air between the plastic film and surface elements of the first roller and/or the second roller, on which the film rests, can be varied differently. Two separating elements are advantageous, as then, for example, the edge areas and the middle area can be subjected to different negative pressures in the axial direction. It is intended to subject the film to higher negative pressure in its edge areas than in its useful area so that damage to the film in the usable area is avoided. These separating elements can be arranged immovably relative to a roller shell. It can be advantageous if these separating elements are arranged so that they can be moved relative to the roll axis.

Separating elements can alternatively or additionally divide, i.e. segment, the inner cavity of one of the aforementioned rollers in the circumferential direction. Such separating elements consequently run parallel to the longitudinal direction of the respective roll. A roll shell can be rotatably mounted around a roll axis, whereby the separating element or elements can be arranged immovably relative to the axis. This makes it possible, for example, to arrange a segment below the surface elements of the first roller and/or the second roller on which the film rests. Consequently, negative pressure and/or an upper pressure is generated only in these surface elements, while essentially ambient pressure prevails in another segment. In this way, the effectiveness of the means for changing the amount of air is increased, since no ambient air is sucked in, but only the amount of air in the relevant areas is changed. At least in the area of the detachment line, the importance of which will be clarified further below in connection with a detachment roller, a segment can be provided which can be subjected to an upper pressure.

This ensures that the film detaches from the first and/or second roller along an actual detachment line that deviates only slightly from the target detachment line.

A segment in the effective area in which the film does not touch down can be subjected to an upper pressure, whereby an airflow from the inside of the roller can be moved outwards. This enables continuous cleaning of the through openings so that increasing contamination and blockage of the through openings over time can be avoided.

At least three separating elements can also be provided in the circumferential direction so that at least three segments can be provided. This results, for example, in a possible arrangement in which a segment pressurized with negative pressure is arranged in the run-up area of the film and a segment pressurized with positive pressure is arranged in the run-out area. Ambient pressure can prevail in all remaining areas. Thus, in the area where the film meets the surface of the roller (run-up area), the entrained air can be extracted, whereby the advantages according to the invention are achieved. In the area where the film leaves the surface again (run-out area), on the other hand, the film can be released from the surface of the roller with an upper pressure, whereby it is avoided that the film loosens at different circumferential angles of the roller seen over its width. Avoiding this effect leads to improved properties, especially the geometric properties of the film. The application of a top pressure can also be used, for example, to clean through openings in the roll shell.

The individual segments can be controlled individually regarding the amount of low and/or high pressure. The control can be carried out by a computer and control unit, which compares measured data from sensors with target data for the control. Such measurement data can be transmitted to the computer and control unit by measurement sensors with which the measurement of the geometric properties of the film, in particular the thickness profile of the film, can be carried out. In this way, control of the upper and/or lower pressure in each segment can be established.

To be able to change the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests, at least one suction device is provided in one embodiment of the invention, with which air can be sucked out of the area in which the plastic film comes into contact with the first and/or the second roller in the transport direction. When the film runs onto the roller, the air entrained by the film gets between the surface elements of the roller and the film. By means of an extraction device, this air can now be extracted in a targeted manner, so that significantly less air is introduced into the mentioned area. In this way, the advantages of the invention can be attained.

Alternatively, or additionally, it can be provided that the area of the roller on which the film does not rest is at least partially covered with a covering device so that the ambient pressure is not applied to the roller surface in this area. If a lower pressure than the ambient pressure is applied in the roller body and/or if a negative pressure is caused by an extraction device arranged close to the roller surface, an intake of air from the environment (false air) can be avoided. This leads to greater effectiveness of the invention, since then only the air entrained by the film, but not any air entrained by the roller, has to be removed. In this context, it is advantageous if the suction device and the cover device are connected to each other and form an assembly.

In a further development, it is provided that the suction device and/or the covering device are divided into sections transverse to the transport direction of the plastic film. It is possible that a suction device extending over the entire width of the plastic film is divided into segments. However, individual sections independent of each other can also be provided. This case is particularly advantageous when it is a question of extracting air in the edges of the plastic film, for example, to minimize the influence of stretching on the geometric properties of the film in its edge area. If such independent sections are provided, it is advantageous if they can be positioned in the transverse direction of the film, i.e. parallel to the axis of the respective roller, by means of a positioning device in order to be able to adapt the sections of the suction device to different film widths.

The individual sections can be controlled separately regarding their suction power. The control can be carried out by a calculation and control unit, which compares measurement data from sensors with target data for the control. Such measurement data can be transmitted to the calculation and control unit by measurement sensors with which the measurement of the geometric properties of the film, in particular the thickness profile of the film, can be carried out. In this way, control of the air extraction can be set up.

An advantageous embodiment of the invention provides that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller, on which the film rests, comprise at least one upper-pressure device, with which the surface of the plastic film facing away from the roller can be acted upon by fluid under upper pressure, in particular air. In this way, the back of the film, i.e. the side facing away from the roller, is subjected to a force applied by the air under upper pressure. This allows the film to be pressed against the roller with a greater force, so that a smaller proportion of the air can enter the area between the film and the roller. Like the suction device described above, the upper-pressure device can be divided into sections, which can additionally be position-adjustable in the direction of the roller axis. A control and/or regulation of the upper pressure and/or the volume flow can also be provided. This can also be done individually, in sections.

If the means for changing the amount of air, which is designed as an upper-pressure device, are arranged in the edge regions of the roller, it can be provided that the fluid with which the edge regions of the roller, on which, in particular, the edge regions of the film rest, is impinged with cold fluid. By “cold fluid” is meant a fluid that has a temperature below 50 degrees Celsius, preferably below 30 degrees Celsius, or preferably below 15 degrees. It is advantageous that the edge areas of the film have a higher stiffness than the useful area of the film before running onto the relevant roller so that the edge areas no longer migrate so strongly inwards. In other words, this reduces the neck-in.

To reduce neck-in, a fixing device can be provided in the area between the first and the second roller. In this case, the fixing device can consist of two disc-like roller pairs, through each of whose roller gaps, an edge of the film runs. In this way, the forces caused by the longitudinal stretching and acting on the edges of the film are compensated or at least reduced. However, fixing devices can also be arranged on or at the first and/or the second roller. For example, these can be surface elements that provide greater adhesion for the film than the other areas of the surface of the roller. A fixing device is displaceable in the transverse direction of the film, i.e. in the axial direction of the first and/or the second roller. In order to increase the adhesion of the film to the first and/or the second roller, it can be provided that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprise at least one electrode with which the plastic film can be electrostatically charged upstream or at the line of contact of the plastic film with the roller. Due to the electrostatic charge of the film, an electrostatic attraction or repulsion force is caused between the film and the roller. If there is an attractive force, i.e. a force directed towards the roller as seen from the film, a small part of the air entrained with the film enters the gap between the film and the roller, so that the film adheres better to the roller. Seen in the axial direction of the respective roller, at least two successive electrodes can be provided whose electrical voltages can be adjusted independently of each other. The electrical voltages can be controllable by means of a control device. In this way, it is possible to change and optimize the line of the impact of the film on the roller. Ideally, this line of impact is a straight line. This variant can help to improve the flatness of the film. In addition, it can be ensured that the edges of the film with a different voltage and thus with a different induced charge adhere to the roller in the same way as the central areas of the film, despite the greater thickness.

In a further embodiment of the invention, an electrode can be combined with an upper-pressure device, such as described above. This makes it possible to electrostatically charge the fluid flowing from the upper-pressure device towards the film, whereby this electrostatic charge can be applied to the film. This again improves the adhesion of the film to the roller.

In connection with an electrode, it can be advantageous if a discharging device is provided downstream of the first and/or downstream of the second roller in the transport direction of the film. This in turn allows the electrostatic charge of the film to be reduced or even completely eliminated, so that a further processing operation, such as winding, is not negatively influenced. It is advantageous if the discharging device is arranged at the target detachment point of the film behind the first and/or second roller or in front of it in the transport direction of the film, in order to have achieved a discharge of the film in the area of the detachment line at the latest. This ensures that areas of the film do not adhere to the roller for different lengths of time, which could lead to flatness errors. An unloading device can, for example, be designed as a grounded roller and/or a brush roller.

A first and/or second roller suitable for the embodiment of the invention may comprise at least partially a metal such as aluminum and/or at least partially plastic, a fiber-reinforced plastic. The aforementioned materials can be used, for example, for a base body of a first and/or a second roller.

Further, the first and/or second roller may comprise a specially machined surface. It has already been described above that the surface may comprise a sintered material. It is conceivable that alternatively or additionally the surface comprises a rubber coating, is at least partially coated with Polytetrafluoroethylene (PTFE), known as Teflon, and/or is at least partially chromium-plated to better adapt the surface properties to the properties of the film. Scratches can be avoided in the case of sensitive films. In the case where sintered material has been used as material for the first and/or second roller, a PFTE may already be embedded before the sintering process to avoid clogging the through openings.

To improve the so-called flatness of a film, at least one detaching roller can be provided, which can be attached to the first and/or the second roller. Ideally, a film should leave the first or the second roller at a target peel line that runs parallel to the longitudinal axis of the roller. The target peel line is located on a surface that forms a common tangential surface of the first and/or second roller and the following roller. In reality, this detachment line is not a straight line, which can lead to errors in the film. A peel roller is positioned in such a way that it forms a nip with the first and/or second roller in the target peel line or in the transport direction of the film behind it. This ensures that the real peel line is aligned with the target peel line. In particular, the peel roller can be displaceable in the circumferential direction of the first and/or second roller.

In a further embodiment of the invention, it is provided that the first and/or the second roller has a shape different from a cylindrical shape. For example, it may be provided that the roller has a conical shape, a concave shape, or a convex shape. With such a design of the roller, various defects, for example, flatness defects, can be counteracted, partly depending on the properties of the film.

In principle, it is advantageous to provide a temperature control device with which the film areas that rest on the first and/or the second roller or are in the section between the first and the second roller can be changed with regard to their temperature. In particular, a temperature control device can be provided with which the said film areas can be brought to different temperatures in the transverse direction, i.e. transversely to their transport direction. For example, the edge areas of the film can be cooled more than the other areas in order to reduce the neck-in.

Such a temperature control device can be integrated with the first and/or the second roller so that the efficiency of this temperature control device is high. In this case, the film itself is tempered by the heat conduction of the roller. For example, the roller body can have at least one pipe running through it, through which a temperature-controlled fluid can be conducted. Such a pipe can extend in the axial direction of the roller to ensure rapid fluid distribution. This pipeline can, for example, be designed as a bore in the roll shell. However, the pipeline can also have a different course, for example helical, to be able to generate a greater heat transfer. In this case, the pipeline can be formed by a tube or a hose, which is arranged inside a hollow roll shell. If the roll shell has several layers, the pipe can also be arranged between two layers. Several pipes are also conceivable to be able to apply different temperatures to different zones of the roller. The temperature-controlled fluid is preferably fed into the roller through one or more rotary unions on the front side.

When designing the first and/or the second roller in combination with sintered materials, it is conceivable to design the roller shell in at least two layers. An inner layer can comprise coarser pores compared to the outer layer so that the fluid can be conducted through these pores. According to the invention, the outer layer can reduce the amount of air between the film and the roller by conducting air away.

In a further variant of the invention, the first and/or the second roller may comprise at least one heating wire that is embedded in a surface layer of the roller. This embedding can be produced in combination with a design of this surface layer from a sintered material.

In an advantageous embodiment of the invention, a temperature control device can be designed as an induction device. A device for providing a magnetic field, which constitutes part of the induction device, can be arranged inside the roller in order to increase the efficiency of this temperature control device. However, to ensure a simpler construction, an arrangement outside the roller is also conceivable.

Alternative or additional temperature control devices based on further physical principles are also conceivable. For example, the temperature of the film can be changed by a device for generating infrared radiation. This is preferably arranged outside one of the rollers. A temperature-controlled gas, which can be conducted from at least one nozzle directly onto the film, can also be provided for temperature control. Air is provided as the gas. Devices for generating infrared radiation and/or a nozzle can also be arranged inside the roller.

In a further advantageous embodiment of the invention, it is provided that the means for changing the amount of air comprise at least one conduit which is arranged inside the roller, inside the roller shell. Furthermore, in this embodiment, connecting conduits are provided which are in fluid-communicating connection with the conduits and mouth with their other ends on the roller surface. Such connecting conduits may again be bores or channels that are formed, for example, in a sintered material. Thus, the air that is between the foil and the roller surface can be discharged through the connecting ducts and the conduits. The ducts can, for example, open in one of the end faces of the respective roller. In particular, the ducts can be axial ducts and/or be designed as axial bores. In the case of a multi-layer structure of the shell of the first and/or the second roller, an inner layer can also be provided which is not air-permeable. Using spacers, an annular space can be provided between the inner layer and the outer layer, in which the air can be collected. The annular space is thus also to be understood as a conduit in the sense described above. An annular space is particularly advantageous if the outer layer is formed from a sintered material.

In an embodiment with axial lines, these can end at the end face of the roller or the end face of an add-on part, such as a journal. A component that is fixed relative to the roller can be placed on this end face, which comprises a hollow space in the shape of a segment of a circle. The ends of the lines can now sweep over this cavity when the rollers rotate. If a negative pressure or an upper pressure is only applied to this cavity, each line is also subjected to negative pressure or an upper pressure, which enters the effective range of the cavity through the rotation of the roller. This pressurization is canceled accordingly when the respective line is outside the effective range again. A cavity pressurized with negative pressure is preferably arranged in such a way that those lines that are in fluid-communicating connection with connecting lines whose ends end in the surface areas on which the film rests can be pressurized with a negative pressure. This prevents too much ambient air from being sucked in. A cavity pressurized with an upper pressure is preferably arranged in such a way that those lines which are in fluid-communicating connection with connecting lines, the ends of which mouth in the surface areas on which the film does not rest and/or at the detachment line of the film, can be pressurized with an upper pressure.

Accordingly, several components of the type described can be provided, for example, one which can be pressurized with an upper pressure and one which can be pressurized with lower pressure.

The above-mentioned task is additionally solved by a method for stretching a plastic film in its transport direction with a first roller, which is driven by a first drive and rotated at a first rotational speed, and with a second roller, which is driven by a second drive and rotated at a second rotational speed. The second rotational speed is greater than the first rotational speed, the second roller is arranged downstream of the first roller in the transport path of the plastic film, and the quantity of air present is changed by means of changing the quantity of air between the plastic film and surface elements of the first roller and/or of the second roller on which the film rests.

With this method, according to the invention, the same advantages can be achieved as have already been described in relation to a device in accordance with the invention.

In another aspect of the invention, a blown film line is provided having at least one extruder for producing a plastic melt, a nozzle head for producing a film tube from the plastic melt, a flattening device for converting the film tube into a double-layer plastic film, a driven draw-off device for drawing off and further transporting the double-layer plastic film, and a winding device for winding up at least one layer of the double-layer plastic film, wherein a stretching device is provided which is designed according to one of claims 1 to 12 and/or according to the above description. Such a blown film line may optionally comprise a calibration device that is arranged upstream of the flattening device in the transport direction. Furthermore, a reversing device can be provided, which is arranged downstream of the extraction device and with which defects, in particular deviations from the average film thickness, can be displaced across the film width. A stretching device according to the invention can now be arranged downstream of the extraction device, for example between the extraction device and an optional reversing device. An arrangement of the stretching device between a reversing device and the winding device is also conceivable. The plastic film can be fed to the stretching device as a film tube laid flat or as a double-layered plastic film cut on one or both sides. A plastic film cut on one side may have been previously unfolded and fed to the stretching device as a single-layer plastic film, which is double-width. A plastic film cut on both sides can be separated into its individual layers, whereby the layers can each be fed to an individual stretching device. A winding device can comprise a winding station for winding an uncut, single-sided cut, or double-sided cut double-layer plastic film. A single-layer plastic film cut on one side—i.e. unfolded—can also be wound. However, two winding points can also be provided for winding one layer each of the previously double-layered plastic film.

Further advantages, features and details of the invention can be seen in the following description, in which various examples of embodiments are explained in detail with reference to the figures. The features mentioned in the claims and in the description may be essential to the invention individually or in any combination of the features mentioned. Within the scope of the entire disclosure, features and details described in connection with the method according to the invention naturally also apply in connection with the stretching device according to the invention, and vice versa in each case, so that reference is or can always be made mutually with respect to the disclosure concerning the individual aspects of the invention. The individual figures show:

FIG. 1 Schematic view of a stretching device according to the invention

FIG. 2 A stretching roller of an embodiment of the invention

FIG. 3 A stretching roller of a further embodiment of the invention

FIG. 4 A stretching roller of a further embodiment of the invention

FIG. 5 An embodiment of the invention with a suction device

FIG. 6 An embodiment of the invention with an electrode

FIG. 7 A blow film line according to the invention with a stretching device

FIG. 8 Another blow film line according to the invention with a stretching device

FIG. 9 An embodiment of the invention with a nozzle

FIG. 10 An embodiment of a device according to the invention with a covering device

FIG. 11 A variant of a covering device

FIG. 12 An embodiment of a device according to the invention

FIG. 13 An embodiment of a device according to the invention

FIG. 1 shows a schematic representation of a stretching device according to the invention. The means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests are not visible in this figure and will be explained in more detail in the following figures. The ribbon-shaped plastic film 101 enters the stretching device 100 in the transport direction T. The plastic film first runs onto one or more rollers. The plastic film first runs onto one or more preheating rollers, of which only one preheating roller 102 is shown. A preheating roller has the task of bringing the film to a predefined temperature. For this purpose, a preheating roller is usually temperature-controlled, whereby a temperature-controlled fluid is often introduced into the preheating roller.

After leaving the preheating roller(s) 102, the film web 101 passes onto a first roller 110, which can generally also be referred to as the first stretching roller 110. This stretching roller is connected to a drive that is not shown, for example, its own electric motor, which drives the roller 110 in rotation at a first rotational speed.

The first roller 110 is preferably associated with a first feed roller 111, which together with the first roller 110 provides an infeed gap for the film. Preferably, the infeed gap or the running path of film 101 is arranged in such a way that film 101 runs in the infeed gap tangentially to the rollers 110, and 111. The roller nip already serves to minimize the air between the stretching roller 110 and the film.

The first roller 110 is also associated with a second feed roller 112, which forms an exit nip with the roller 110. The second feed roller 112 can be adjustable in the circumferential direction of the first roller 110. The second feed roller 112 serves to ensure that the film leaves the first roller 110 along a line that runs parallel to the axial direction of the roller 110.

As seen in the transport direction T of film 101, a second roller 120 is arranged downstream, which can be referred to as the second stretching roller 120. This stretching roller 120 is also connected to a further drive which is not shown, for example, its own electric motor, which drives the roller 120 in rotation at a second rotational speed. The second rotational speed is greater than the first rotational speed, whereby the second stretching roller then has a greater circumferential speed than the first stretching roller. This results in film 101 being stretched between the outlet nip of the first roller 110 and the inlet edge of the second roller 120 in the ratio of the circumferential speeds in its transport direction. The distance between the outlet nip and the inlet edge is often referred to as the stretching nip.

It is possible that the first roller 110 and the second roller 120 can be moved relative to each other. This allows the stretching gap to be influenced. A change in the stretching gap can influence the properties of the film.

The second roller 120 is preferably assigned a third feed roller 121, which together with the first roller 120 provides a second infeed gap for the film. Preferably, the infeed nip or the running path of film 101 is arranged in such a way that film 101 runs in the infeed nip tangentially to the rollers 120, and 121. The roller nip already serves to minimize the air between the stretch roller 120 and the film.

In the case that the drafting gap is small, the third feed roller 121 would collide with the roller 110, so in this case, the third feed roller had to be swung off.

The second roller 120 is also optionally associated with a fourth feed roller 122, which forms an exit nip with the roller 120. The second feed roller 122 can also be adjustable in the circumferential direction of the first roller 120. The second feed roller 122 serves to ensure that the film leaves the first roller 120 along a line that runs parallel to the axial direction of roller 120.

In principle, one or more feed rollers can also be dispensed within a stretching device according to the invention. Nevertheless, an infeed nip or outfeed nip can be mentioned. This is to be understood as the line along which the film is applied to the stretching roller or along which the film is released from the stretching roller.

Further stretching rollers can be provided, in particular with one or two feed rollers in each case, whereby two stretching rollers arranged directly one after the other are driven in each case in such a way that the stretching roller arranged downstream in each case has a higher circumferential speed than the preceding stretching roller.

Downstream of the stretching rollers 110, and 120, a cooling roller 130 is arranged, with which film 101 can be cooled again so that the new molecular orientation within the film resulting from the stretching solidifies.

With reference to FIG. 2 , an embodiment of the stretching device according to the invention is explained below. A perspective view of a stretching roller is shown. In the example shown, it is the stretching roller 120, which comprises a roller shell 140 and end faces, of which the end face 141 is visible. The stretching roller has a shaft, and an axle or axle stubs. Of the aforementioned components, only part 142 projecting from the end face 141 is visible. The roller shell 140 now includes depressions 143, which in the present example is shown as a circumferential groove. Further examples of depressions have been given earlier in the description of the invention. The characteristic of depressions is that they only extend into the roll shell, but do not extend through the roll shell 140 into the interior space of the roll defined by the roll shell and the end faces.

FIG. 3 is similar to FIG. 2 , but here the roller 120 comprises through openings 150. The exact design of the through openings has already been described above. The through openings extend into the interior of the roller and thus form a fluid-communicating connection between the interior and the surroundings of the roller 120. Instead of through openings, openings can also protrude from the surface, which only comprises cavities that are open to the outside, for example, the open area of porous material.

The interior of the roller 120 can optionally be pressurized with negative pressure. For this purpose, the axis, the shaft, or the stub shaft can be provided with fluid lines. In the case of a shaft, fluid lines comprise a rotary union. The fluid line of the axis, shaft, or stub shaft is connected to another fluid line, such as a hose 151, which leads to a vacuum source.

FIG. 4 shows further possible features of a roller 120 which has through openings. Here, the interior space, which is now visible due to the missing representation of the roller shell, can be divided into several individual spaces with partition walls, in the present example two partition walls 160, 161. This makes it possible to apply different air pressures to the different individual chambers. In the present case with two individual rooms, one of them can, for example, be subjected to negative pressure, while in the second individual room no upper or lower pressure can be applied, so the ambient pressure prevails here.

FIG. 5 shows an example of an embodiment in which a suction device 165, which extends in the transverse direction with respect to the transport direction of film 101, is arranged upstream of the stretching roller, here in particular the stretching roller 120. This suction device comprises a closed box 166 with one or more suction openings 167. In particular, the box can be subjected to a negative pressure so that air from the inlet gap enters the suction device through the suction openings in the direction of the arrow L and is discharged.

FIG. 6 shows a further example of a stretching device according to the invention, in which an electrode 170 is arranged upstream of the inlet gap of a stretching roller, in the example shown the stretching roller 120. This electrode can be subjected to an electrical potential that differs from the electrical potential of film 101. This results in an electrical charge of the film, which leads to the film being additionally held on the stretching roller with an electrical force.

FIG. 7 shows apparatus 1 for the production of a film tube, namely a blown film line, which initially comprises at least one extruder 2, with which, for example, plastic present in granular form can be plasticized. Via pipe 3, the plastic melt thus produced is fed to an extrusion tool 4, which can also be referred to as a nozzle head, from which this melt is transferred into a film tube 6, so that this melt stream can be drawn out of an annular gap 5, which is not visible in this figure, in the drawing-off direction z. Now there is a film tube 6 which has not yet solidified. This is inflated from the inside by a slight upper pressure so that it has a larger diameter inside the optional calibration device 7. The film tube is solidified by a tempering device 8, which is often also called a cooling ring because of its ring-like design enclosing the film tube.

After passing through the calibrating device, film tube 6 enters the active area of a flat laying device 9, in which the circular film tube is converted into an elliptical cross-section with increasing eccentricity until it finally forms a double-layered plastic film in the area of influence of the take-off rollers 10, which are joined together at their sides.

The flattening device is rotatably arranged, whereby the axis of rotation is essentially aligned with the hose axis 11, which is indicated in FIG. 1 by a dashed line. The rotatability of the flattening device is indicated by arrow 12.

FIG. 7 also shows a reversing device 15, which has the task of guiding the flattened film tube from the flattening device to the stationary roller 16 without causing damage.

Downstream of the reversing device 15, a stretching device 100 according to the invention is now arranged, which has already been explained in connection with FIGS. 1 to 6 and the further description. The stretching device shown in FIG. 7 corresponds to the one shown in FIG. 1 . It should also be noted that a cutting device can be positioned upstream of the stretching device 100, with which one or two edges of the fold can be cut open or cut off. In addition, a separating device can be provided with which the double-layer plastic film can be divided into one or more single-layer films.

Arrow 17 indicates that this film tube, after passing through the stretching device 100, is guided for further processing, which is not specified in more detail here.

FIG. 8 shows a further embodiment of a blown film line according to the invention, in which the stretching device 100 is now arranged between the removal rollers 10 and the reversing device 15. It should be noted that the rollers within the stretching device are now arranged in such a sequence that the plastic film can be transported from the bottom to the top.

FIG. 9 shows a further example of a stretching device according to the invention, in which at least one nozzle 180 is arranged upstream of the inlet gap of a stretching roller, in the example shown the stretching roller 120. Through this nozzle, the film can impinge on the side facing away from the roller with a pressurized fluid 181, in particular compressed air. This ensures that the film is printed with a force against the roller 120 so that less air is drawn into the area between the film and the roller 120 by the movement of the film.

Numerous possible features of the invention are indicated in the description and in the claims. Even if a free combinability of different features is not always mentioned, this is to be regarded as also disclosed, provided that no contradictions are caused.

FIG. 10 shows a further embodiment of a device according to the invention, in which a covering device 185 is shown on one of the stretching rollers. Such a covering device can also be provided on any other stretching roller. With this covering device 185, the angular area of the stretching roller that is not wrapped by the film can be covered, so that a vacuum that is applied to areas of the roller surface and/or in its interior is not disturbed and is maintained as far as possible. The covering device can interact with the stretching roller without contact. The covering device can, for example, be a bent sheet. The covering device can extend in the axial direction over almost the entire length of the roller. Seals can be provided at the lateral edges of the cover device, which additionally seal the area between the roller and the cover device to further improve the reduction of false air. The cover device can be formed from several cover elements that can be moved against each other. This makes it possible to optimally adjust the cover element even with different film widths and with different wrap angles of the film.

FIG. 11 shows a variant of a covering device, whereby the covering device comprises at least one circulating belt 186, which is also guided by the stretching roller. At least two deflection rollers 187 are provided for guiding and/or driving the circulating belt. The deflection rollers can be displaceable in the circumferential direction and/or in the radial direction of the stretching roller.

To minimize the effect of false air intake described in connection with FIGS. 10 and 11 , which is to be avoided, it can be provided according to the embodiment example of FIG. 12 that the fourth feed roller 122 is dimensioned and/or positioned in such a way that the smallest possible area is created which is not wrapped by the film. Since this area that is not wrapped depends on a number of parameters, it is not possible to make any specific statements about the position and dimension of the roller 122. It is possible for a skilled person to determine such details.

To minimize the effect of false air intake described in connection with FIGS. 10 and 11 , which is to be avoided, a first guide roller 190 and a second guide roller 191 can also be provided, as shown in FIG. 13 . Each of the guide rollers can be in contact with the film. The first guide roller is arranged upstream of the stretching roller in question, and the second guide roller is arranged downstream of the stretching roller. The first guide roller may be identical to the second feed roller 112. A sealing element 192 is arranged between both guide rollers 190 and 191, which preferably contacts the circumferential surfaces of the guide rollers. The resulting cavity consisting of the stretching roller, the film, the guide rollers, and the sealing element is closed, in particular, by seals arranged on the front side of the cavity, but not shown.

List of references 100 Stretching device 101 Plastic film 102 Preheating roller 110 First roller 111 First feed roller 112 Second feed roller 120 Second stretching roller 121 Third feed roller 122 Second feed roller 130 Cooling roller 140 Roll shell 141 Front face 142 Part protruding from the face 141 143 Depressions 150 Through opening 151 Tube 160 Partition wall 161 Partition wall 165 Suction device 166 Closed box 167 Suction opening 170 Electrode 180 Duse 181 Fluid 185 Covering device 186 Circulating belt 187 Deflection rollers 190 First guide roller 191 Second guide roller 192 Sealing element 1 Device for the production of a film tube 2 Extruder 3 Line 4 Extrusion tool 5 Not visible annular gap 6 Not yet solidified film tube 7 Optional calibration device 8 Tempering device 9 Flattening device 10 Removal rollers 11 Tube axis 12 Arrow to illustrate the rotatability of the flattening device 13 14 15 Reversing device 16 Stationary roller 17 Arrow indicating further processing T Transport direction Z Removal direction 

1. A stretching device for stretching a plastic film in its transport direction, having a first roller that can be driven by a first drive and can be rotated at a first rotational speed, and having a second roller that can be driven by a second drive and can be rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film, characterized by means for changing the amount of air between the plastic film and surface elements of the first roller and/or of the second roller on which the film rests.
 2. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprise depressions in the surface of the first roller and/or the second roller, the air being receivable in the depressions.
 3. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprised through openings in the surface of the first roller and/or the second roller said through openings providing a fluid-communicating connection between the outer surface and a cavity within the first roller and/or the second roller.
 4. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprised through openings in the surface of the first roller and/or the second roller, wherein at least part of the through openings can be provided by a porous material, in particular by a microporous material.
 5. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprise through openings in the surface of the first roller and/or the second roller, at least a part of the through openings being provided with a porous material, in particular by a microporous material, the porous material being a sintered material, in particular, a sintered metal.
 6. A stretching device according to claim 1, characterized in that the means for varying the amount of air comprised through openings in at least part of a roll shell of the first roll and/or the second roll, at least part of the through openings being formed by bores.
 7. A stretching device according to claim 1, characterized in that the first and/or the second roller comprise at least one cavity which can be subjected to an air pressure that is lower than the ambient pressure.
 8. A stretching device according to claim 1, characterized in that the first and/or the second roller comprise at least one cavity which is divided into at least two segments in the axial direction and/or in the circumferential direction by means of at least one separating element, wherein at least one segment can be subjected to an air pressure which is reduced or increased in comparison with the ambient pressure.
 9. A stretching device according to claim 1, characterized in that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprises at least one suction device with which air can be sucked out of the region in which the plastic film comes into contact with the first and/or the second roller in the transport direction.
 10. A stretching device according to claim 1, characterized in that the suction device is divided into sections transversely to the transport direction of the plastic film.
 11. A stretching device according to claim 1, characterized in that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller, on which the film rests, comprises at least one upper-pressure device, with which the surface of the plastic film facing away from the roller can be acted upon with air under upper pressure.
 12. A stretching device according to claim 11, characterized in that the means for changing the amount of air between the plastic film and surface elements of the first roller and/or the second roller on which the film rests comprise at least one electrode with which the plastic film can be electrostatically charged upstream or at the line of contact of the plastic film with the roller.
 13. A method for stretching a plastic film in its transport direction with a first roller, which is driven by a first drive and rotated at a first rotational speed, and with a second roller, which is driven by a second drive and rotated at a second rotational speed, the second rotational speed being greater than the first rotational speed, the second roller being arranged downstream of the first roller in the transport path of the plastic film, characterized in that the quantity of air present between the plastic film and surface elements of the first roller and/or of the second roller, on which the film rests, is changed by means of changing the quantity of air.
 14. A blow film installation comprising at least one extruder for producing a plastic melt, a die head for producing a film tube, a flattening device for converting the film tube into a double-layer plastic film, a driven extraction device for drawing off and further transporting the double-layer plastic film, and a winding device for winding up at least one layer of the double-layer plastic film, characterized by at least one stretching device according to claim
 1. 